1. Field of the Invention
The present invention relates to the recovery of hydrocarbon oils from petroleum residue and, more particularly, to an improved process for selectively extracting heavy hydrocarbons from asphalt-containing petroliferous oils.
2. Brief Description of the Prior Art
As known in the art, crude oils are conventionally subjected to atmospheric distillation to remove light materials such as gas oils, kerosenes, gasolines, lighter petroleum hydrocarbons and the like. The residue from this atmospheric distillation is then typically subjected to vacuum distillation to produce a vacuum gas-oil type distillate and a vacuum reduced residuum which contains asphalt, resins and heavy hydrocarbon oil components. The recovery of the latter, i.e., the heavy hydrocarbon oils and their upgrading, has become particularly significant in view of the acute shortages and high cost of whole crudes.
In this regard, in recent years considerable effort has been expended to develop improved techniques or processes for recovering the aforesaid hydrocarbon oils from asphaltic petroleum residua. To a considerable extent known processes involve extraction of the oil with a deasphalting solvent, as e.g., propane, and thereafter separating and recovering the oil components from the solvent. Thus in U.S. Pat. No. 2,950,244, there is disclosed a process for the extraction of petroleum residue wherein the asphalt containing feedstock is contacted with a liquified normally gaseous solvent, i.e., propane, which is maintained at a temperature of from between 100.degree. to 250.degree. F. and at a pressure sufficient to maintain the solvent in the liquid phase.
While propane is conventionally employed in such known deasphalting operations, further solvents have been proposed. In U.S. Pat. No. 4,054,512, an asphalt-containing mineral oil is deasphalted by contacting the oil with liquid hydrogen sulfide. The use of the liquid neopentane, maintained at a temperature of from about 0.degree. to 250.degree. F., as the deasphalting solvent is disclosed in U.S. Pat. No. 3,334,043. In U.S. Pat. No. 2,337,448, a heavy residuum is deasphalted by contacting the feedstock with a solvent such as ethane, ethylene, propane, propylene, butane, butylene, isobutane and mixtures thereof.
Multi-stage solvent extraction techniques involving the use of one or more solvents are also known. Thus in U.S. Pat. No. 3,658,695 there is disclosed a liquid solvent extracting process involving the use of two or more extraction zones. In this process, the deasphalted oil and solvent, recovered from the first extraction zone, is subjected to the addition of a further solvent in a second zone, the latter being maintained at a higher or increased temperature. U.S. Pat. No. 4,017,383 discloses a further multi-stage deasphalting process with the basic concept in this technique involving the recovery of the solvent from the extracted hydrocarbons in a series of two or more pressure stages. The solvents employed in this process comprise liquified low molecular weight hydrocarbons such as propane and isobutane.
While many such proposals are known and have achieved, at least to some extent, commercial significance, in general they suffer from a number of known disadvantages including the plugging of the equipment by the asphalt-resin fraction, low yields of the deasphalted heavy hydrocarbon oil, complex multi-stage operating procedures and the like. The present invention is directed to an advance and improvement over such prior known techniques.